The most important features of electrostatic photography wherein an electrostatic latent image formed in such systems as electrophotography, electrostatic recording, ink jet recording, cathode ray tube recording is converted into a visible image through a development processing, resides in that the electrostatic photographic system is simple, rapid and inexpensive as compared with silver salt photography. Further, the most important feature of liquid developers for electrostatic photography resides in that an image of high quality can be reproduced with a high resolving power as compared with dry developers for electrostatic photography.
Generally, liquid developers for electrostatic photography are obtained by dispersing organic or inorganic pigments or dyes such as carbon black, nigrosine, phthalocyanine blue, etc. and natural or synthetic resins such as alkyd resins, acrylic resins, rosin, synthetic rubbers, etc. in a liquid having high electric insulating properties and a low dielectric constant such as petroleum aliphatic hydrocarbons, and further adding a charge controlling agent such as metal soaps, lecithin, linseed oil, higher fatty acids or vinylpyrrolidone-containing polymers.
In the compositions of these liquid developers, colored grains are used in order to convert an electrostatic latent image into a visible image, and resin grains are used to improve the fixation of the image and to keep the strength of the image. Grains comprising a coloring component as well as a fixing component may also be used.
Charge controlling agents for imparting positive charge or negative charge to these grains are used to thereby form electricity detecting toner grains.
The electricity detecting toner grains must have strong and stable charges. Particularly, in recent years, line originals and halftone originals as well as the originals of continuous tone images have been markedly increased as originals to be duplicated (copied).
The chargeability of the toner grains is very important to reproduce the faithfully duplicated (copied) images of such fine originals. When the chargeability is insufficient, the desired image density cannot be obtained, and images formed tend to have a flow defect (so-called streak) and further the deposition of the toner grains on non-image areas (fog) tends to occur.
Charge controlling agents for reducing or eliminating such a phenomenon as described above have been developed, and such agents include semi-alkylamide compounds of maleic acid copolymers as disclosed in JP-B-49-26594 (the term "JP-B" as used herein means an "examined Japanese patent publication") (U.S. Pat. No. 4,062,789), JP-A-60-179750 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and metal salts of N,N-dialkylaminoalkanecarboxylic acids and metals salts of N,N-dialkylaminoalkanesulfonic acids as disclosed in JP-A-60-21056.
A change in an image quality and sensitivity caused by repeated development of a large number of electrophotographic materials (hereinafter sometimes referred to as plates) must be minimized as much as possible to keep the practical features, i.e., simplicity, rapidness and low cost of electrostatic photography as well as high image quality of the liquid developers. Generally, when development is repeatedly carried out a great number of times, changes in the image quality such as the image density or the gradation and the sensitivity may occur with a reduction in the concentration of the toner grains and with a change in the composition of the developer. When the image quality is changed, high image quality which is one of the advantages of the liquid developer cannot be obtained. When sensitivity is changed, the control of an exposure amount is required and simplicity and rapidness which are the advantages of electrophotography cannot be attained. Further, in developing a great number of plates repeatedly with the same developer, if image quality and sensitivity are greatly changed, it is necessary that a developer is frequently replaced with a new one, and thus the advantages of simplicity, rapidness and low cost are reduced.
When a great number of plates, for example, 1000 plates or more, are developed with conventional liquid developers, image quality is greatly changed, and hence developing apparatuses are so designed that the apparatuses are provided with an automatic controlling device to keep the concentration of toner grains in the developing tank constant. However, such a means for keeping the concentration of toner grains constant causes other problems such as an increased cost for apparatuses and complicated structures of the apparatuses, thereby causing machine troubles and time-consuming works for maintaining the apparatuses.
In order to overcome these problems, a proposed liquid developer containing further branched aliphatic alcohols having not less than 12 carbon atoms has been proposed in JP-B-63-55063.
Further, JP-A-57-210384 discloses a method wherein the concentration of a developer used for replenishment during the repeated use is higher than that of a developer used at starting of the development, and JP-A-48-90236 and JP-A-64-32278 disclose a method wherein an amount of a charge controlling agent used is reduced in a developer for replenishment and the resulting developer is replenished.
However, these techniques are still insufficient with respect to the characteristics required for stably duplicating highly fine original images including continuous tone images presently required over a long period of time. Further, it is still more required that the original images are stably reproduced by electrostatic photography even when environmental conditions are greatly changed to such as low temperature and low humidity conditions or high temperature and high humidity conditions. Furthermore, even in electrostatic photography using a liquid developer, a requirement for easy maintenance comparative to the dry developer system has recently been desired.
Accordingly, developments of a liquid developer having excellent repeated use characteristics and a duplicating method using the same are important matters.